Pentagon chief Chuck Hagel is ordering a review of rules on hairstyles for female troops after a wave of complaints charged the US military's policies discriminated against black women.

In a letter obtained by AFP Wednesday, Hagel wrote to lawmakers promising that each of the armed services would look at hairstyle regulations that apply to African-American women "to ensure standards are fair and respectful of our diverse force."

Sixteen female lawmakers from the Congressional Black Caucus had raised objections over hairstyle regulations saying the rules unfairly singled out black women in uniform.

And a former member of the Georgia National Guard, Jasmine Jacobs, launched a White House petition against the rules that attracted more than 17,500 signatures along with a huge volume of online comments.

The complaints came after the US Army announced updated appearance and grooming rules in March that cited prohibited hairstyles.

The rules banned dreadlocks and some other styles that are worn predominantly by African-American women, deeming them "unkempt."

"The use of words like 'unkempt' and 'matted' when referring to traditional hairstyles worn by women of color are offensive and biased," the female lawmakers said in a letter to Hagel.

Black female troops said the rules were patently unfair because they discouraged styles using their natural hair while permitting some wigs or hair weaves.

One of the lawmakers who had raised the issue, Marcia Fudge of Ohio, welcomed Hagel's letter as a "thoughtful response."